The simple answer is not all ARs are legal in Mass. We had a Assault weapons ban in this state, since 1994. This ban mimics the expired federal ban. Which means; no flash hiders, bayonet mounts, high capacity magazines, or collapsable stocks. Which means if you buy a factory model out of state, it must comply with the Mass regulations. Most AR compaines still sell a few models, with all the evil features removed, though useally you will have to special order it through a dealer. You can see why Assulat rifles are not very popular in Mass. Check out www.goal.org for more detailed info.

I should add that you can own Pre ban models, with all the evil features as long as it was made before 1994. It is also legal to own Pre ban (pre 1994) high capacity magazines.